The present invention relates to a liquid crystal display unit, and more particularly to a method for driving such a display which contributes significantly to improving the reliability of an active matrix liquid crystal display.
The conventional methods for driving an active matrix liquid crystal display have been disclosed in U.S. Pat. Nos. 4,906,984 (Takeda), 4,186,395 (Fujita), 4,870398 (Bos), 5,010,327 (Wakita), 5,010,328 (Morris), and JP-A-No. 62-54230. For example, the prior art disclosed in JP-A-No. 62-54230 is designed to invert a polarity of a scan voltage to be applied into each scan line for the purpose of reducing flicker and an amplitude of a signal voltage.
As is well known, frame is the time period of applying a signal of one complete picture, and consists of two fields in interlaced scanning for display.
As one prior art as shown in FIGS. 10a and 10b, the prior art is designed to have each scan line composed of a liquid crystal pixel capacitance C.sub.LC and a connection circuit made of a thin film transistor and a signal line so that a scan voltage V.sub.G is applied to the scan line, a signal voltage V.sub.D is applied to the signal line, and a common voltage V.sub.com is applied to an electrode located as opposed to the liquid crystal capacitance C.sub.LC. The scan voltage V.sub.G is arranged to alternately change the polarity, positive or negative at each scan line (1H).
The above-mentioned driving method is required to invert the signal voltage and the common voltage at each scan line like alternate current. This requirement makes it difficult to design voltage-alternating circuits for both of the signal voltage and the common voltage. As an example, consider an active matrix liquid crystal display having a diagonal of 14 inches and pixels of 1120.times.1024. It needs a time of about 16 .mu.s for scanning one line and a load capacitance of about 0.3 .mu.F as viewed from an opposite electrode. This means that the voltage-alternating circuit for the common voltage needs to have quite a low output impedance. It is, therefore, quite difficult to design the voltage-alternating circuit for the common voltage.